"rectifier type" AC voltmeters

W

Walter Harley

Guest
I was looking at some AC panel voltmeters in my friendly neighborhood
surplus shop the other day. There seem to be two kinds, "rectifier type"
and "iron vane". They make rectifier type meters that go as low as 1V FS -
for instance, see http://www.simpsonelectric.com/pdf/webpdfe/Round%20VAC.pdf
towards the bottom of the page.

How do these "rectifier type" meters work? Specifically, how do they cope
with the rectifier's forward voltage drop, when the fullscale voltage is so
low?
 
"Walter Harley" <walterh@cafewalterNOSPAM.com> wrote in message
news:vuk268htoirc41@corp.supernews.com...
I was looking at some AC panel voltmeters in my friendly neighborhood
surplus shop the other day. There seem to be two kinds, "rectifier type"
and "iron vane". They make rectifier type meters that go as low as 1V
FS -
for instance, see
http://www.simpsonelectric.com/pdf/webpdfe/Round%20VAC.pdf
towards the bottom of the page.

How do these "rectifier type" meters work? Specifically, how do they cope
with the rectifier's forward voltage drop, when the fullscale voltage is
so
low?

Germanium diodes would help.
Click to expand...
Perhaps the forward voltage drop behaves nonlinearly at low voltage?
If so, then the meter movement could have a compensating nonlinearity.
Or, a truncated meter scale.

I had undergraduate physics lab before they threw out the Bakelite meters,
and there was a "lab tip" -- meters were traditionally calibrated for
maximum accuracy at 70% of fullscale reading.
Experimenters were cautioned to construct experiments so that readings were
obtained near the point of maximum accuracy for the meter in use.
The loss of accuracy at the low end might seem intolerable by modern
standards.
 
Robert Morein wrote:
"Walter Harley" <walterh@cafewalterNOSPAM.com> wrote in message
news:vuk268htoirc41@corp.supernews.com...
I was looking at some AC panel voltmeters in my friendly neighborhood
surplus shop the other day. There seem to be two kinds, "rectifier type"
and "iron vane". They make rectifier type meters that go as low as 1V
FS -
for instance, see
http://www.simpsonelectric.com/pdf/webpdfe/Round%20VAC.pdf
towards the bottom of the page.

How do these "rectifier type" meters work? Specifically, how do they cope
with the rectifier's forward voltage drop, when the fullscale voltage is
so
low?

Germanium diodes would help.
Perhaps the forward voltage drop behaves nonlinearly at low voltage?
If so, then the meter movement could have a compensating nonlinearity.
Or, a truncated meter scale.

I had undergraduate physics lab before they threw out the Bakelite meters,
and there was a "lab tip" -- meters were traditionally calibrated for
maximum accuracy at 70% of fullscale reading.
Experimenters were cautioned to construct experiments so that readings were
obtained near the point of maximum accuracy for the meter in use.
The loss of accuracy at the low end might seem intolerable by modern
standards.
Click to expand...
There were (also?) a number of (old "Bakelite") Weston meters that
were guaranteed to indicate within 1/2% of full scale that had a mirror
behind the needle, and were at least as good as 1% of reading(or better)
all te way down to zero.
In most cases, these were far better than modern VOMs, and comparable
to 3-digit DMMs.
Tossing out such reliable and dependable instruments was stupid.
 
"Walter Harley" <walterh@cafewalterNOSPAM.com> wrote in message
news:vuk268htoirc41@corp.supernews.com...
I was looking at some AC panel voltmeters in my friendly
neighborhood
surplus shop the other day. There seem to be two kinds,
"rectifier type"
and "iron vane". They make rectifier type meters that go as low
as 1V
FS -
for instance, see
http://www.simpsonelectric.com/pdf/webpdfe/Round%20VAC.pdf
towards the bottom of the page.

How do these "rectifier type" meters work? Specifically, how do
they cope
with the rectifier's forward voltage drop, when the fullscale
voltage is
so
low?
Click to expand...
It's not at all difficult to make an accurate AC voltmeter down to 1
volt f.s., or even 1 millivolt f.s. if you have an external source of
power to run the meter. If the meter must work using only the power
that can be drawn from the metered source, 1 volt f.s. is not going to
work very well.

If you never have to read a voltage lower than 1 volt AC, it's
possible to draw power from the source and create a DC operating
voltage sufficient to run the meter. I certainly wouldn't do it, if I
could avoid it.

Norm Strong
 
normanstrong wrote:
"Walter Harley" <walterh@cafewalterNOSPAM.com> wrote in message
news:vuk268htoirc41@corp.supernews.com...
I was looking at some AC panel voltmeters in my friendly
neighborhood
surplus shop the other day. There seem to be two kinds,
"rectifier type"
and "iron vane". They make rectifier type meters that go as low
as 1V
FS -
for instance, see
http://www.simpsonelectric.com/pdf/webpdfe/Round%20VAC.pdf
towards the bottom of the page.

How do these "rectifier type" meters work? Specifically, how do
they cope
with the rectifier's forward voltage drop, when the fullscale
voltage is
so
low?

It's not at all difficult to make an accurate AC voltmeter down to 1
volt f.s., or even 1 millivolt f.s. if you have an external source of
power to run the meter. If the meter must work using only the power
that can be drawn from the metered source, 1 volt f.s. is not going to
work very well.

If you never have to read a voltage lower than 1 volt AC, it's
possible to draw power from the source and create a DC operating
voltage sufficient to run the meter. I certainly wouldn't do it, if I
could avoid it.

Norm Strong
Click to expand...
Be advised that the full scale sensitivity of most analog meters is in
the range of 50 to 250mV, so that a 1 volt scale is trivial.
I have a Triplett 5 microamp movement, which has 16K meter resistance;
calculate the full scale voltage sensitivity: 80mV. The common 1mA
movements will be higher.
Furthermore, i spoke of reading Weston meters way down from full scale
and still getting 1% (or better) accuracy of *reading* (NOT of full
scale, which would be worse).
 
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